This news article was originally written in Spanish. It has been automatically translated for your convenience. Reasonable efforts have been made to provide an accurate translation, however, no automated translation is perfect nor is it intended to replace a human translator. The original article in Spanish can be viewed at Aditivos en automoción
Additives for use in automotive
The market demand has led to a growing interest in plastics designed for special applications, incorporating additives and loads. Automotive is one of the markets that most requests to plastics. This is the subject of a paper presented by Loredana merchant, of Iberian LVL, in the seventh days of plastic automotive organized by the Centre Spanish of plastics. The day was by title thermoplastic compounds for the automotive sector.
In the automotive sector, the most commonly used thermoplastic compounds are autolubricados compounds and conductive materials. The composite self-lubricated of use when the thermoplastic material is exposed to friction and wear. Used additives give thermoplastics for a special dynamic behavior. The characteristics of these materials are measured using the following parameters: coefficient of friction (static and dynamic), the factor of wear and the PV limit or thermal instability. The main additives used in these compounds are the PTFE (Polytetrafluoroethylene), silicone, carbon fiber, fiberglass, graphite and molybdenum bisulfide.
The PTFE dispersed evenly on the thermoplastic resin creates a surface layer with a low coefficient of friction and a high resistance to pressure. The best performance is obtained with rates of 10 to 20 percent, as higher percentages increase wear and tear. However, the use of this additive may lead to the effect slip/stick, consisting of an unstable behavior during boot.
The silicone tends to migrate to the surface of the thermoplastic parts injected, creating a lubricant protector. The Silicon reduces the coefficient of friction, get a factor of low wear and a higher PV limit.
A cost-effective solution is to merge the properties of PTFE and silicone, which reduces slip/stick and provides effective lubrication at high speeds.
The incorporation of a thermoplastic compound carbon fibers improving the mechanical performance of the piece at high temperatures and increases the PV limit of the material, without increasing wear.
It also increases the thermal conductivity and the injected piece is therefore able to better dissipate the heat generated by friction. It also improves the electrical conductivity.
Used traditionally as a lubricant for metal parts, although it is also used in thermoplastics, particularly in applications of pieces that are submerged in water. The graphite is suitable for contact with drinking water.
The effect of graphite is similar to the of the carbon fiber, with the exception that does not improve the mechanical properties as it does the carbon fibre and Yes improving substantially the anti-static properties. The coefficient of friction of a resin loaded with graphite is between that of a polymer without load and on a compound, PTFE and silicone.
A cost-effective solution is to merge the properties of PTFE and silicone, which reduces slip/stick and provides effective lubrication at high speeds
In the injected parts, the outer layer remains in the amorphous phase, thereby reducing the compound tribological properties. The main effect of the molybdenum bisulfide is to act as agent nucleante: forms a crystalline layer on the surface which improves the properties of the compound.
This type of load is used in semi-crystalline polymers.
The incorporation of the fiberglass improves the mechanical and thermal performance (such as carbon fiber) and raises the limit PV. Although it can also increase the wear of the piece. This negative effect can be balanced with additives lubricants.
Glass fiber increase the coefficient of friction.
Conductive thermoplastics are employed to reduce the risk of electrostatic discharges in automotive. Additives used for increasing the conductivity in this sector are carbon fiber, graphite, stainless steel fibers and the black coal.
Graphite (natural or synthetic) It is used due to its properties lubricants a non higher proportion of 30 per cent. Added to amorphous materials, increases dimensional stability.
Carbon fibers, derived from the poliacriolinito, rayon, or pitch, They are used to increase the rigidity of the material. They have the advantage that wear less than glass fibers.
They stainless steel fibers (8 - 12µm in diameter) have an optimal resistance and it resilience. The reduction of electromagnetic interference (EMI) are produced by absorption and dissipation. The compound that incorporate this additive are easy to process.
Finally, the black coal (carbon black), is used fibres of carbon with appearance of a fine powder (13-60 nm).
The best autolubricante
The parameters that are used to measure the benefits of a thermoplastic compound high are as follows:
- Coefficient of friction. It is the resistance to the movement of a body on a surface. -The force necessary to begin the movement - can be static, or dynamic - force necessary to keep the movement. In the case of the thermoplastic materials the dynamic coefficient is greater than the static.
- Limit PV. Specifies the maximum product of pressure and speed beyond which appears a thermal instability followed by a failure of the component.
- Factor of wear. It is a ratio that indicates the resistance to wear and tear on the material. The factor of wear is measured with the same instrument that is used to set the PV limit and is based on the loss of weight of the worn out piece.
On the other hand, the surface finish affects on wear surface, depending on the roughness (to more roughness, most wear) and used material (wear of aluminum is greater than that of steel)
Alejandro Devís and Miriam Gallur, Itene
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